1. Field of the Invention
The present invention relates to a spindle motor and a pivot assembly composed of steel-made parts excellent in the following characteristics: wear resistance, corrosion resistance and high workability.
2. Description of the Related Art
A high degree of rotating accuracy is demanded in a spindle motor and a pivot assembly or the like applied to computer peripheral devices, such as a hard disk drive (HDD), a floppy disk drive (FDD) and a printer. Further, office automation devices, such as a facsimile terminal equipment, are also required to satisfy the above demand. A component for a shaft of a spindle motor, and a shaft and a sleeve of a pivot assembly requires a high-dimensional accuracy to achieve such a rotating accuracy. The usage of a highly workable material is expected to improve the dimensional accuracy of the parts. Because the computer peripheral devices and the office automation devices are employed under various environments depending on users, the component material for the shaft of the spindle motor, and the shaft and the sleeve of the pivot assembly or the like need to secure the certain value of wear and corrosion resistance.
For satisfying the above conditions, various kinds of stainless steels or heat-treated steels have been conventionally applied for manufacturing the shaft of the spindle motor, the shaft and the sleeve of the pivot assembly or the like. However, an austenitic stainless steel having corrosion resistance has been expensive and has not completely satisfied the requirements of hardness and wear resistance. For a martensitic stainless steel hardness and wear resistance become improved through a heat treatment; however, corrosion resistance and workability become degraded, so that a surface treatment is required for improving corrosion resistance and for suppressing out-gas. Besides, the addition of lead (Pb) has been required to make the steel further workable.
Still further, a more workable steel is expected by increasing the required sulfur content. The heat treatment increases its hardness, wear resistance while retaining corrosion resistance. Out-gas suppression could not be achieved without employing the surface treatment. However, the following extra cost has not been avoided due to the additional process of the surface treatment: the additional manufacturing cost for the surface treatment and for environmental countermeasures over the surface treatment.
Although a high manganese steel has been developed for the above materials, the high manganese steel""s wear resistance and hardness are unsuitable for the invention. Therefore, the following countermeasures have been taken: a heat treatment to the high manganese steel for improving wear resistance and/or the addition of sulfur to materials making the workability more preferable or the like. However, these countermeasures result in the generation of the out-gas of sulfur and sulfur compounds (hydrogen sulfide, sulfur dioxide, etc.) out of the materials when finished products are employed. Consequently, the reliability of the shaft of the spindle motor, the shaft and the sleeve of the pivot assembly or the like will be lowered.
The invention in the light of the above-described problems has an object to achieve a qualitative improvement for a computer, peripheral devices thereto and office automation devices by manufacturing a shaft of a spindle motor and a shaft and a sleeve of a pivot assembly with a material high in wear resistance, corrosion resistance and workability and in which the generation of the out-gas of sulfur and sulfur compounds is equivalent to or less than conventional austenitic steel or both heat and surface treated martensitic steel.
In order to achieve the above-described object, a spindle motor according to the present invention is provided with a shaft composed of a material high in wear resistance, corrosion resistance and workability and in which the generation of the out gas of sulfur and sulfur compounds is suppressed low.
Further, in the spindle motor according to the present invention, a steel material including 0.20 wt % of C, 0.35 wt % of Si, 8.00 wt % of Mn, higher than 0 and equal to or less than 0.05 wt % of P, 0.15 wt % of S, higher than 0 and equal to or less than 2.00 wt % of Ni and 14.00 wt % of Cr and the remainder substantially being Fe is employed.
Further, the spindle motor according to the invention does not contain Pb.
Further, in order to achieve the above-described object, a pivot assembly according to the invention is provided with a sleeve and a shaft composed of a material high in wear resistance, corrosion resistance and workability and in which the generation of the out gas of sulfur and sulfur compounds is equivalent to or less than conventional austenitic steel or both heat and surface treated martensitic steel.
Still further, in the pivot assembly according to the present invention, a steel material including 0.20 wt % of C, 0.35 wt % of Si, 8.00 wt % of Mn, higher than 0 and equal to or less than 0.05 wt % of P, 0.15 wt % of S, higher than 0 and equal to or less than 2.00 wt % of Ni and 14.00 wt % of Cr and the remainder substantially being Fe is employed.
Further, the pivot assembly according to the invention does not contain Pb.